function x_next = RK4(f, x, h)
    % 四阶 Runge-Kutta 单步积分
    k1     = f(x);
    k2     = f(x + 0.5*h*k1);
    k3     = f(x + 0.5*h*k2);
    k4     = f(x +    h*k3);
    x_next = x + (h/6)*(k1 + 2*k2 + 2*k3 + k4);
end